Why is the "Save for Web" file size way off the actual size?

Question

In Photoshop, the Save for Web dialogue does not give an accurate size for the file. For instance:

The original file size is 2.6MB (checked with Cmd + I on a Mac). When I open it in Photoshop and use File → Save for Web, it tells me that the file size (Original) is 6.73MB. It happens in all my photos.

Why does Cmd + I on a Mac 2.6MB and Photoshop's Save for Web 6.7MB. In a PRACTICAL way, what number should I trust for web design?

It seems that if I upload the file to a server the important number is Cmd + I. Is that right? if so, does the number that gives Photoshop have any PRACTICAL utility?

Answer 1

Keep in mind that if your operating system supports file and drive compression, then it will show a slightly different file size to you than a software that has loaded that file and is looking at its raw loaded size. The discrepancy is mostly due to how the operating system handles size calculations.

For example, on Windows when you look at the properties of a file, you will see something to the effect of size of the file, and then size on disk. The difference is a number of things, for example cluster size, type of formatting be it NTFS, exFat, FAT, FAT32, so on can lead to difference size measurements for the same file. Hope that helps.

Answer 2

Your original image is likely compressed as JPEG on disk. The "original" size that Photoshop is telling you is the probably the "uncompressed" decoded JPEG in memory while you are working on it.

The diff between the saved file PS and Finder size is a filesystem thing. The correct size in terms of data is the one PS is telling you. The size written on disk is constrained by the filesystem's block size, hence the small difference.

Answer 3

You can use tools like imageOptim on mac to remove useless data like os metadata. https://imageoptim.com/fr.html

Answer 4

I don't think it's an error so much as the difference between how files types are interpreted by Adobe or Windows or Mac. I've seen this too. Just like the difference between programs interpreting how much space is available on a drive.

Answer 5

The difference comes from having two common understandings of the word "kilobyte".

The SI prefixes kilo, mega, giga all refer to powers of ten. A kilometer is 1000 meters. However, in computing it has been common for people to say kilo when referring to 2^10 (1024). Mega is used for 2^20 (1048576). For KB, that's close, but the higher prefixes things get further and further from the non-computing meaning.

For a long time this has caused confusion. Hard drive manufacturers have long used the base-10 values to describe the size of their drives, which can upset someone when their "500GB" hard drive shows up looking smaller on their computer, if it reports the base-2 values.

ISO standardized separate prefixes to use when discussing base-2 numbers, in ISO/IEC IEC 80000-13:2008. Instead of kilo, mega, and giga, we have kibi, mebi, and gibi. The standard abbreviations are KiB, MiB, etc.

Why does this matter?

Photoshop is reporting values using the base-2 values. OS X switched to base-10 sizes in Snow Leopard. (You can switch it back if you want.) So 2KB in Photoshop means 2048 bytes, which is 2.048KB in the Finder.

There's still a very small discrepancy between the value that PS reports that I believe is due to the extra metadata and header flags that PS adds during save.